Basic steps in the CVD process
R ligand
metal atom
Precursor
Transport
Gas Phase Reactions
R
RR
Adsorption
R
R
R
+
Adsorption
Desorption
R
R
R
Decomposition
reactions Difusion
Nucleation
R
R
R
R +
Heated substrate
Diffusion of
precursor to
a surface
Diffusion of
products from
surface
1
Cold-Wall CVD Reactor
2
Hot-Wall CVD Reactor
3
CVD Kinetics
Deposition depends on the sequence of events:
(1) Diffusion of precursor to surface
(2) Adsorption of precursor at surface
(3) Chemical reaction at surface
(4) Desorption of byproducts from surface
(5) Diffusion of byproducts from surface
The slowest event will be the rate-determining step
4
CVD Kinetics
Growth Rate Model
F1 = precursor flux from bulk of gas to substrate surface
F1 = hG ⋅ (CG - CS)
hG = mass-transfer coefficient hG = D / 
D = gas diffusion constant D = Do T 3/2 / P
 = boundary layer thickness (related to gas velocity)
CG, CS = precursor conc. at bulk of gas and at substrate surface
(conc. gradient – driving force for diffusion)
F2 = flux consumed in film-growth reaction (rate of chemical reaction)
F2 = kS ⋅ CS
kS = surface-reaction rate constant: kS = A exp (Ea/kT)
Steady state
F1 = F2 = F

5
CVD Kinetics
Growth Rate Model
F1 = F2 (rate of transport = rate of reaction)
hG ⋅ (CG - CS) = kS ⋅ CS
CS = CG / (1 + kS/hG)
F = kS hG CG / (kS + hG)
Growth rate (thickness growth rate)
dy / dt = F / 
y = film thickness
 = atomic density of film
Steady state
F1 = F2 = F
GS
G
hk
C
dt
dy
11
11



6
Growth Rate
GS
G
hk
C
dt
dy
11
11


Growth rate is determined by:
a) Concentration CG of a precursor in bulk of gas mixture
b) By the smaller of hG and kS
kS << hG = Surface reaction limited dy/dt  exp(Ea/kT)
hG << kS = Mass transport limited dy/dt  T3/2
When temperature is low, surface reaction rate is slow, and
excess of reactants is available = the reaction is surface reaction
limited
Above a certain temperature all source gas molecules react
immediately = the reaction is then in mass-transport limited
regime (also diffusion limited and supply limited regime) 7
Deposition rate vs. Temperature
ln (dy / dt)
GS
G
hk
C
dt
dy
11
11



8
Deposition rate vs. Temperature
A = Surface reaction limited
B = Mass transport limited 9
Growth Rate Dependence on Flow Velocity
F1 = hG ⋅ (CG - CS)
hG = mass-transfer
coefficient
hG = D / 
 = boundary layer
thickness
At constant T
Low flow rate U
 large boundary layer thickness 
 slow mass-transfer
10
Precursor Volatility









12
0
1
2 11
ln
TTR
H
p
p subl
Clausius-Clapeyron Equation
Thermogravimetric Analysis
11
Al
CH3
H
CH3
H
H
below 330 oC
-Hydride Elimination
CH3
CH3
H
H
Al
H
Al
CH3
CH3
H
H
H2
Al
CH3
H
CH3
H
H
above 330 oC
-Methyl Elimination
CH3
HH
H
Al
CH3
Al
CH3
HH
H
H2
C
CVD of Al
TIBA = Triisobutylaluminum
Clean deposition
Carbon impurities
12
Aluminum
2.27  cm, easily etched, Al dissolves in Si
GaAs + Al  AlAs + Ga
Gas diffusion barriers, Al on polypropylene, food packaging = chip bags,
party balloons, high optical reflectivity = mirrors
CVD of Al
Al deposits selectively on Al surfaces, not on SiO2
Laser-induced nucleation
248 nm only surface adsorbates pyrolysed
193 nm gas phase reactions, loss of spatial selectivity control
TMA = Trimethylaluminum
Large carbon incorporation, Al4C3, RF plasma, laser
Al2(CH3)6  1/2 Al4C3 + 9/2 CH4 under N2
Al2(CH3)6 + 3 H2  2 Al + 6 CH4 under H2
DMAH = Dimethylaluminum hydride
Ligand redistribution
[(CH3)2AlH]3  (CH3)3Al + AlH3
AlH3  Al + H2 at 280 C, low carbon incorporation
13
Al
H
H
Al
H
H
N
N
H
H
CH3
H3C
CH3
H3C
CH3
H3C
Al
H
H
N
H
CH3
CH3
H3C
Al H
H
N
H
CH3
CH3
H3C
CH3
H3C
H3C
N
CVD of Al
TMAA = Trimethylammine-alane
(CH3)3N-AlH3  Al + (CH3)3N + 3/2 H2
Decomposition mechanism of
TMAA on Al – below 100 C
14
Al
H
H
B
H
H
N
H
H
CH3
CH3
H3C
Al
H
H
B
H
H
H
H
H
H
B
B
H
H
H
H
CVD of Al
Aluminoboranes
(CH3)3N-BH3 + Al + 3/2 H2
15
3/2 B2H6 + Al + 3/2 H2
2 WF6 + 3 Si  2 W + 3 SiF4
WF6 + 3 H2  W + 6 HF
WF6 + 3/2 SiH4  W + 3 H2 + 3/2 SiF4
W(CO)6  W + 6 CO
CVD of W
Tungsten
5.6  cm, a high resistance to electromigration, the highest mp
of all metals 3410 C
Tungsten hexafluoride (WF6) is the heaviest known gas at room
temperature and pressure, density = 13 g/L
Purity 99.999%, extremely corrosive and toxic
16
O O
H3
C CH3
HH
KETO
ENOL
O O
H3
C CH3
H
H
O O
H3
C CH3
H
O O
H3
C CH3
- H+
Diketonate Precursors
Mononuclear
Polynuclear
17
Copper(II) hexafluoroacetylacetonate
Excellent volatility
CF3 groups (low polarizibility)
Vapor pressure of 0.06 Torr at r. t.
Low decomposition temperature
Stability in air, low toxicity
Commercial availability
Deposition on metal surfaces (Cu, Ag, Ta)
The first step can already occur at -150 °C
= a dissociation of the precursor molecules on the surface
An electron transfer from a metal substrate to the single occupied
HOMO, which has an anti-bonding character with respect to copper
dxy and oxygen p orbitals, weakens the Cu-O bonds and facilitates
their fission
CVD of Cu
18
Scheme I
-150 o C
CF3
CF3F3 C
F3 C
O
O
O
Cu
O
e
O
C u
O
F3 C CF3
+
F3 C CF3
O O
F3 C CF3
OOH
C
CO + CF 3
2 H (ads)H 2 (g)
Cu o
>250 o C
CF3C
C
O
>100 o C
CVD of Cu
Copper(II) hexafluoroacetylacetonate
19
Growth rate of Cu films deposited from Cu(hfacac)2 with 10 torr of H2
CVD of Cu
Surface reaction limited
Mass transport limited
20
Cu(I) Precursors
Disproportionation to Cu(0) and Cu(II)
2 Cu(diketonate)Ln  Cu + Cu(diketonate)2 + n L
O
Cu
O
R R
L
O
Cu
O
R R
LL
L: PMe3, PEt3, CO, CNt
Bu,
SiMe3
CVD of Cu
21
CVD of YF3 from Y(hfacac)3 Complex
22
Diamond films
Activating gas-phase carbon-containing precursor molecules:
• Thermal (e.g., hot filament)
• Plasma (D.C., R.F., or microwave)
• Combustion flame (oxyacetylene or plasma torches)
CVD of Diamond
23
Experimental conditions:
Temperature 1000 - 1400 K
The precursor gas diluted in an excess of H2
Typical CH4 mixing ratio ~1-2 vol%
Deposited films are polycrystalline
Film quality:
• the ratio of sp3 (diamond) to sp2-bonded (graphite) carbon
• the composition (e.g., C-C versus C-H bond content)
• the crystallinity
Combustion methods: high rates (100-1000 µm/hr), small, localised areas,
poor quality films
Hot filament and plasma methods: slower growth rates (0.1-10 µm/hr),
high quality films
CVD of Diamond
24
Hydrogen atoms generated by activation (thermally or via electron
bombardment)
H-atoms play a number of crucial roles in the CVD process:
H abstraction reactions with hydrocarbons, highly reactive radicals: CH3
(stable hydrocarbon molecules do not react to cause diamond growth)
Radicals diffuse to the substrate surface and form C-C bonds to propagate
the diamond lattice
H-atoms terminate the 'dangling' carbon bonds on the growing diamond
surface - prevent cross-linking and reconstructing to a graphite-like
surface
Atomic hydrogen etches both diamond and graphite - under CVD
conditions, the rate of diamond growth exceeds its etch rate whilst for
graphite the converse is true
This is the basis for the preferential deposition of diamond rather than
graphite
CVD of Diamond
25
Diamond initially nucleates as individual microcrystals,
which then grow larger until they coalesce into a
continuous film
Enhanced nucleation by ion bombardment:
• Damage the surface - more nucleation sites
• Implant ions into the lattice
• Form a carbide interlayer - glue, promotes diamond growth, aids
adhesion
Diamond laser window
CVD of Diamond
26
Substrates: metals, alloys, and pure elements:
Little or no C Solubility or Reaction: Cu, Sn, Pb, Ag, and Au, Ge, sapphire,
diamond, graphite
C Diffusion: Pt, Pd, Rh, Fe, Ni, and Ti
The substrate acts as a carbon sink, deposited carbon dissolves into the
metal surface, large amounts of C transported into the bulk,
a temporary decrease in the surface C concentration, delaying the onset of
nucleation
Carbide Formation: Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Co, Ni, Y, Al
B, Si, SiO2, quartz, Si3N4 also form carbide layers
SiC, WC, and TiC
CVD of Diamond
27
Thermal management - a heat sink for laser diodes, microwave integrated circuits,
active devices mounted on diamond can be packed more tightly without overheating
Cutting tools - an abrasive, a coating on cutting tool inserts
CVD diamond-coated tools have a longer life, cut faster and provide a better finish than
conventional WC tool bits
Wear Resistant Coatings - protect mechanical parts, reduce lubrication
gearboxes, engines, and transmissions
Optics - protective coatings for infrared optics in harsh environments
ZnS, ZnSe, Ge: excellent IR transmission but brittle
the flatness of the surface, roughness causes attenuation and scattering of the IR
signal
Electronic devices - doping, an insulator converted into a semiconductor
p-doping: B2H6 incorporates B into the lattice
doping with atoms larger than C very difficult, n-dopants such as P or As, cannot be
used for diamond, alternative dopants, such as Li
Applications of Diamond Films
28
CVD of ZnO
SEM of the ZnO film
Bar = 1 μm
Hexagonal ZnO PDF 79-0208
500 C, 1 h
29
Aminoalcoholates TG
XRD
Laser-Enhaced CVD
Si(O2CCH3)4  SiO2 + 2 O(OCCH3)2
ArF laser
Substrate
Heated source
Heater
Vacuum chamber
Vacuum
Pump
30
Aerosol-Assited CVD
31
Carried out at atmospheric pressure
Non-volatile precursors
Formation of droplets in ultrasonic nebulizer / atomizer
Precursor aerosol delivered by the stream of carrier gas to reactor
Solvent evaporation
Pulsed Injection MOCVD
32
Low-pressure hot-wall reactor - vacuum 10 Torr
A pulsed liquid injection system - electromagnetic injector
A metal-organic precursor dissolved in a solvent (DME)
Precise micro-doses (several L, frequency 2 Hz)
A hot (200 C) evaporation zone - flash evaporation of micro-doses
A mixture of precursor and solvent vapors carried into the deposition
zone with a Ar:O2 (4:1) gas mixture
Depositions on sapphire-C substrates at 350 - 900 C
Sn(OtBu)2(PyTFP)2 (1)
Sn(OtBu)2(DMOTFP)2 (2)
Sn(Bu)2(acac)2
Atomic Layer Deposition ALD
Film growth by cyclic process
4 steps:
1) Exposition by 1st precursor
2) Cleaning of the reaction
chamber
3) Exposition by 2nd precursor
4) Cleaning of the reaction
chamber
A method for the deposition of thin films
Cycle repetitions until desired film thickness is reached
1 cycle: 0.5 s – several sec. thickness 0.1- 3 Å 33
Atomic Layer Deposition ALD
Requires high reactivity
Self-Limiting growth mechanism
Formation of a monolayer
Control of film thickness and composition
Carried out at room temperature
Reactor walls inactive – no reactive layer
Separate loading of reactive precursors –
no gas-phase reactions
Deposition on large surface area
Highly conformal coverage of surface
Precursor transport to the reaction zone
does not have to be highly uniform
34
Comparison of ALD and CVD
ALD
• Highly reactive precursors
• Precursors react separately
on the substrate
• Precursors must not
decompose at process
temperature
• Uniformity ensured by the
saturation mechanism
• Thickness control by the
number of reaction cycles
• Surplus precursor dosing
acceptable
CVD
• Less reactive precursors
• Precursors react at the
same time on the substrate
• Precursors can decompose
at process temperature
• Uniformity requires uniform
flux of reactant and T
• Thickness control by
precise process control and
monitoring of T, flow, time
• Precursor dosing important
35
ALD vs. CVD vs. PVD Comparison
36AR = the ratio of width to depth
Conformal coverage improves
ALD Precursor Properties
Selection of suitable combination of precursors
Molecular size influences film thickness
Gases, volatile liquids, solids with high vapor pressure
Typical precursors:
Metallic - halogenides (chlorides), alkyls, alkoxides, organometallics
(cyclopentadienyl complexes), alkyl amides
Nonmetallic - water, hydrogen peroxide, ozone, hydrides, ammonia,
hydrazine, amines
Thermally stable
Must react with surface centers (hydroxyl groups on oxide surface)
Thermodynamics – Kinetics – Mechanisms
37
Examples of ALD
High-permitivity Oxides
Al(CH3)3 / H2O
ZrCl4 / H2O
HfCl4 / H2O
DRAM capacitors
(Ba,Sr)TiO3 – Sr and Ba cyclopentadienyl compounds
and water as precursors
Nitrides of transition metals
TiN - TiCl4 and NH3
TaN - TaCl5 / Zn / NH3
WN - WF6 and NH3
WCxNy 38
Examples of ALD
Metallic films
Difficult by ALD: metal surface has no reaction sites,
low reactivity with reducing agents
W - WF6 and Si2H6
Ru, Pt - organometallic precursors and oxygen
applies to all precious metals capable of catalytic
dissociation of O2
Ni, Cu – metal oxide reduction by hydrogen radicals formed in
plasma
Al – direct reduction of AlMe3 by H radicals from plasma
39
ALD of SiO2 and Al2O3 Films
Precursors:
• Trimethylalane
• Tris(tert-butoxy)silanol
Deposition of amorphous SiO2 and nanolaminates of Al2O3
32 monolayers in 1 cycle
Applications:
• microelectronics
• optical filters
• protective layers (against diffusion, oxidation, corrosion)
40
Si O
H
O
O
O
ALD of SiO2 and Al2O3 Films
Step A
Step B
41
ALD of SiO2 and Al2O3 Films
C, D: alkoxide - siloxide exchange
42
ALD of SiO2 and Al2O3 Films
E: elimination of isobutene = formation of -OH
Si
O
O
O
O
M
CH3
CH3
H
H
H
Si
O
O
O
OH
M
H3C CH3
H H
+
-H elimination
43
ALD of SiO2 and Al2O3 Films
F: elimination of butanol = condensation
G: elimination of water = condensation
OH OH
OH OH
44
ALD of SiO2 and Al2O3 Films
Repeat Step A
45
Molecular Layer Deposition MLD
Sequential, self-limiting reactions A and B for MLD
growth using two homobifunctional reactants
46
Molecular Layer Deposition MLD
ABC TypeAB Type
47
Diols vs. Polyols
Homobifunctional precursors can react twice with the AlCH3*
surface species, double reactions lead to a loss of reactive
surface sites and decreasing growth rate
48
AB Lewis Acid-Lewis Base Reactions
49